Organic electroluminescent (EL) devices or organic light-emitting diodes (OLEDs) are electronic devices that emit light in response to an applied potential. Tang et al. in Applied Physics Letters 51, p913, 1987; Journal of Applied Physics, 65, p3610, 1989; and commonly assigned U.S. Pat. No. 4,769,292 demonstrated highly efficient OLEDs. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved.
OLEDs are thin-film devices comprising an anode, a cathode, and an organic EL element disposed between the anode and the cathode. In operation, an electrical voltage is applied between the anode and the cathode causing electrons to inject from the cathode and holes to inject from the anode. When properly constructed, the injected electrons and holes recombine in the light emitting layer within the organic EL element and the recombination of these charge carriers causes light to emit from the device. Typically, the organic EL element is about 100˜500 nm in thickness, the voltage applied between the electrodes is about 3˜10 volts, and the operating current is about 1˜20 mA/cm2.
Because of the small separation between the anode and the cathode, the OLED devices are prone to shorting defects. Pinholes, cracks, steps in the structure of OLED devices, and roughness of the coatings, etc. can cause direct contacts between the anode and the cathode or to cause the organic layer thickness to be smaller in these defective areas. These defective areas provide low resistance pathways for the current to flow causing less or, in the extreme cases, no current to flow through the organic EL element. The luminous output of the OLED devices is thereby reduced or eradicated. In a multi-pixel display device, the shorting defects could result in dead pixels that do not emit light or emit below average intensity of light causing reduced display quality. In lighting or other low resolution applications, the shorting defects could result in a significant fraction of area non-functional. Because of the concerns on shorting defects, the fabrication of OLED devices is typically done in clean rooms. But even a clean environment cannot be effective in eliminating the shorting defects. In many cases the thickness of the organic layers is also increased to beyond what is actually needed for functioning devices in order to increase the separation between the two electrodes to reduce the number of shorting defects. These approaches add costs to OLED device manufacturing, and even with these approaches the shorting defects cannot be totally eliminated.
JP2002100483A discloses a method to reduce the shorting defect due to local protrusions of crystalline transparent conductive films of an anode by depositing an amorphous transparent conductive film over the crystalline transparent conductive film. It alleged that the smooth surface of the amorphous film could prevent the local protrusions from the crystalline films from forming shorting defects or dark spots in the OLED device. The effectiveness of the method is doubtful since the vacuum deposition process used to produce the amorphous transparent conductive films does not have leveling functions and the surface of the amorphous transparent conductive films is expected to replicate that of the underlying crystalline transparent conductive films. Furthermore, the method does not address the pinhole problems due to dust particles, flakes, structural discontinuities, or other causes that are prevalent in OLED manufacturing processes.
JP2002208479A discloses a method to reduce shorting defects by laminating an intermediate resistor film made of a transparent metal oxide of which, the film thickness is 10 nm–10 μm, the resistance in the direction of film thickness is 0.01–2 Ω-cm2, and the ionization energy at the surface of the resistor film is 5.1 eV or more, on the whole or partial of light emission area on a positive electrode or a negative electrode formed into transparent electrode pattern which is formed on a transparent substrate made of glass or resin. While the method has its merits, the specified resistivity range can not effectively reduce leakage due to shorting in many OLED displays or devices. Furthermore, the ionization energy requirement severely limits the choice of materials and it does not guarantee appropriate hole injection that is known to be critical to achieving good performance and lifetime in OLED devices. Furthermore, the high ionization energy materials can not provide electron injection and therefore cannot be applied between the cathode and the organic light emitting layers. It is often desirable to apply the resistive film between the cathode material and the organic light emitting layers or to apply the resistive film both between the cathode and the organic light emitting materials and between the anode and the organic light emitting materials.